This document discusses the history and types of firewalls. It describes early circuit-level firewalls developed at AT&T and DEC in the 1980s. Packet filtering firewalls screening network traffic based on IP and TCP/UDP headers were introduced by Cisco in the early 1990s. Application layer firewalls called proxy firewalls were developed in the late 1990s, screening traffic based on application protocols like HTTP, FTP, and SMTP. The document also discusses stateful packet inspection, demilitarized zones (DMZs), and common firewall threats.